Electrical power is traditionally generated with fixed frequency (FF) and applied to many different types of linear and nonlinear loads in the aerospace industry and other industries. Adjustable speed drives provide energy conservation and higher efficiencies at light load conditions at the expense of requiring power conditioning equipment to provide a variable voltage—variable frequency (VVVF) conditioned power for the speed control of the drive system. While utilities and general industry use a 50/60 Hz electrical system for the distribution of electric power, the aerospace industry has been using a 400 Hz system for its military and commercial applications. Recently, these traditional fixed frequency (FF, i.e., 400-Hz) generators are being gradually replaced by variable frequency (VF) generators, which deliver power at frequencies between 320 and 800 Hz. The reasons for transition to variable frequency include efficiency, weight, and economics. To maintain constant frequency, a constant speed of the alternator must be maintained. This is not an easy task when the generator is on the shaft of an aircraft's engine. In order to maintain a constant frequency, additional subsystems are required which are heavy and expensive and reduce the overall efficiency of the power system. Alternatively, a variable frequency power source for aerospace can be used. However, this approach poses a fundamental problem because certain types of loads are sensitive to variations in the supply frequency.
Many types of loads, whether active or passive, are sensitive to variations in the supply frequency and voltage magnitude. One example of such a load is an aerospace pump/fan application. In a variable frequency (VF) power system for aerospace, the frequency variation may have a ration of 2:1 or more. Since power for a typical fan/pump load is proportional to the cube of frequency, in this VF power system, the load is subjected to a power demand that is eight times its rating and is consequently damaged. Additionally, when the voltage magnitude of the source decreases, the load tries to draw more current if a constant power is maintained. This results in overheating of the load and can cause eventual damage. Therefore, to alleviate the negative impact of variable voltage variable frequency sources of power on these sensitive loads, advanced power conversion equipment capable of taking variable voltage and variable frequency AC power or unregulated DC power at the input are required to provide synchronous fixed or variable frequency conditioned power as required for numerous high performance “more electric loads” for aerospace and other industrial applications.
Many different power conversion solutions have been presented in the technical literature to alleviate the problems associated with the frequency sensitivity of certain loads to VF power by first converting the VF input power to an intermediate DC power, and then using inverters to condition the DC power to the desired fixed or variable frequency as required by the load in on-off control mode or adjustable speed/frequency systems. Examples of these systems include:                Six pulse passive rectification along with passive filters;        12, 18, and 24 pulse rectification using 2, 3, or 4 passive three-phase rectifiers along with multi-phase auto-transformers and additional filters for harmonic cancellation;        Single channel active rectification; and        Multiple channel active rectification.        
In variable frequency systems, most prior art solutions use simple three-phase passive diode rectification for AC-DC power conversion to avoid the complex implementation of traditional active rectifiers. The use of active rectification for aerospace and other industrial applications has been hindered by the complexity of the design, inherent failure modes, and excessive cost of these systems. Recently, active rectifier solutions have been suggested for aerospace applications. In particular, the technique disclosed in U.S. Pat. No. 6,038,152 developed by D. E. Baker suffers from the following problems:                The control method does not provide a regulated DC bus voltage;        The control method can not be used to provide a higher value than the normal three phase diode rectification level;        The proposed method uses a fixed four pulse gating pattern, which cannot provide the benefits of a closed-loop gating pattern control with high dynamic performance and ability to control the fundamental component of the AC system voltage in real time due to a very low switching frequency. Therefore this method is not suitable for high dynamic performance DC-AC loads such as adjustable speed drives;        Implementation of memory-based gating patterns are based on feed-forward pre-calculated switching patterns and would not allow for real time error correction in case of deviation from the assumed system model; and        Furthermore, this reference suggests that implementation of an active rectifier with a 10 kHz switching frequency to meet power quality requirements of aerospace for VF systems is impractical due to excessive power losses and can not be achieved.        
Prior art active rectification for different types of AC-DC converters to meet power quality and proposes a digital active rectifier for fixed frequency AC power system applications suffers from certain drawbacks. More particularly, the control structure/algorithm is complex, tuning of the controller parameters is not straightforward and the implementation is not cost effective as it requires high digital signal processing throughput. Furthermore, large filter components and excessive thermal management is required due to a very high switching frequency of 20 kHz for the controlled devices, which significantly increases the cost and weight of the overall system.
Lack of a robust control method and proper synchronization suitable for the wide frequency variation range of 320-800 Hz for aerospace variable frequency systems has hindered the acceptance of prior art active rectifiers/inverters as a viable solution for aerospace high power conversion/utilization applications. These traditional power conversion systems suffer from poor reliability, excessive cost and weight and lower power density. Prior art power conversion equipment is not easily scalable, flexible or configurable to support modular building blocks for cost-effective next generation power conversion equipment with much improved power density, reliability and minimal thermal management requirements. Finally, the packaging, manufacturing and maintenance of most of the existing power conversion equipment is tedious, time consuming and expensive.
Optimized implementation of a control structure/algorithm requires an accurate and easy to implement Phase Lock Loop (PLL) to measure the variable frequency of the system for successful synchronization of the power conditioning equipment to the source (AC-DC) or load (DC-AC) or to both (AC-DC-AC). Once the system frequency is measured, as is well known in the technical literature, real-time rotating reference frame angles are calculated which are then used for standard rotating vector (abc-to-alpha/beta) and stationary time-invariant reference frame (alpha/beta-to-d/q) transformations and vice-versa (i.e., transformation back to abc from d-q stationary reference frame). These transformations are needed for generation of gating patterns for the controlled devices of the power conditioning system (i.e., AC-DC active rectifier or DC-AC inverter) and proper regulation and closed-loop control of system variables such as DC bus voltage and power factor correction.
In power electronics based systems, the output of the PLL is a clock signal, which provides real-time adjustment of the sampling frequency for the A/D conversion system as the frequency (f) of the AC-side is changing. In addition, real time phase angle information is required for rotating vector to stationary reference frame and vice versa transformations. In most PLLs the real-time phase delay angles are obtained by integration of the angular frequency (2 Π f) with respect to time.
Many different types of analog and digital PLL systems have been proposed and used for power electronic systems. Analog PLLs have been well modeled and developed by linear control theory, starting from a well-defined model in the continuous-time-domain. However, power electronics systems are sampled data systems and are non-linear. Linear-control theory and modeling can be only used for very high sampling rates. A high switching frequency requires high digital signal processing throughput, is expensive to implement and results in poor efficiency due to excessive switching frequency losses. This demands bulky and expensive thermal management systems. For practical reasons a very high switching frequency is not affordable due to these problems and the fact that present high power semiconductor devices such as IGBTs, mainly used for power conditioning systems, are limited to switching frequencies well below 20 kHz. The development and implementation of nonlinear Digital Phase Lock Loop (DPLL) systems have been hindered for power electronic systems due to the complexity of nonlinear and discrete control theory.
Prior art PLLs are not suitable for a wide variable frequency power system due to the following reasons:                Limited frequency tracking capability;        Difficulty in designing the loop filter and tuning the parameters of the required controllers (in most cases, a proportional-integral, i.e., a PI controller); and        Stability analysis and implementation is not straightforward due to the measurement time-delays and the fact that the nonlinear system is only modeled as a second order system.Additionally, the prior art implementation of analog PLLs is subject to op-amp offsets, drift and parameter variations. The digital PLLs require high sampling rate, require a lot of on-line calculations and are difficult to implement.        
There are many components, devices, equipment and systems required to make up an electrical power conversion system for motor controls applications. A stand-alone motor controller includes an inverter with all the associated controls, protection circuitry, thermal management, input and output connectors and is properly packaged in a chassis. The cost, weight, size, efficiency and reliability of a motor controller is a complex function of power rating, duty cycle, cooling medium, environmental requirements and is also significantly impacted by how these main subassemblies/functions are realized and partitioned with respect to each other and integrated together in a package for ease of manufacturing and maintenance.
Conventional power electronics based motor controllers include the following main subassemblies/functions:                Logic power supply;        Power electronics controller;        Control strategy/algorithm;        Power pass inverter (and rectifier if AC-DC-AC) devices/module;        Signal measurement (current, voltage temp, speed, etc.) and isolation;        Gate driver;        Power interconnect;        Logic interconnect;        Power sequencing, protection coordination and fault tolerance circuitry;        EMI and power quality filters for input and output;        Thermal management;        DC link capacitor;        Type and size of motor drive (DC Machine, Induction Machine, PMSM, wound field SM, SRM etc.);        Controls I/O; and        Chassis.The following requirements/considerations would also have a major impact on the design and manufacture of the motor controller:        Method of speed control (hall effect/resolver or sensorless method);        Level of integration with the electromechanical system (motor/generator);        Centralized versus distributed control circuitry; and        Environmental requirements.        
The conventional aerospace motor controller practice is very refined. However, these power conversion technologies can not be effectively used for future commercial transport applications because they suffer from:                Excessive weight yielding a very low power density;        High cost due to custom made parts and approach;        Low reliability due to excessive number of component count and lack of proper health monitoring and protection coordination;        Low efficiency due to excessive power losses in the power electronic devices and associated filters; and        Large size due to excessive partitioned functional blocks and poor design partitioning/integration.        
Some of the reasons why such “conventional” motor controls technologies can suffer from these problems include the following:                Separate “logic power supply” with many voltage levels utilized;        “Centralized Controls” concept is used with multiple control boards and logic/power interconnects for controlling the motor using mainly analog circuitry or non-optimized custom-made digital circuits with relatively low digital signal processing throughput Control structure/algorithm not optimized for bi-directional and wide variable frequency power systems;        Power pass inverter realized by discrete devices and are custom made and very expensive;        Bulky sensors and expensive methods used for signal measurement (current, voltage temp, speed, etc.) and high voltage signal isolation;        Gate driver circuit design is custom made for each application and does not have all the necessary protection and diagnostics functions to effectively handle failure modes at the device level;        Extensive use of bulky and expensive power interconnect (e.g., Bus Bars);        DC Bus of 270 VDC and AC voltage of 115V, 400 Hz. The main limitation being the fact that the DC bus is obtained by natural three phase diode rectification and can be significantly lower or higher and the AC system has to be 400 Hz fixed frequency;        Extensive use of expensive and fragile logic interconnect;        Limited BIT and lack of comprehensive power sequencing (such as soft-start/stop and ride-through) protection coordination, health monitoring and prognostics and fault tolerance circuitry results in poor functional performance and/or reliability;        Bulky and expensive EMI and power quality filters for input and output;        Bulky and expensive custom made thermal management;        Low energy density high voltage capacitors are custom made and used as bulk “DC link” capacitors. These are temperature dependent, expensive and difficult to package;        The developed motor controls are not flexible to accommodate different types and sizes of motor drives (Induction Machine, Permanent Magnet Synchronous Machine (PMSM), and wound field SM); and        Large size chassis required due to low power density and excessive volume required to package the different subassemblies.        